Gated p/i/n structures, so called tunnel field effect transistors (TFET), are prominent for their abrupt switching behaviour [0] as the injection mechanism is band to band tunneling (BTBT) which consequently liberates them from the 60 mV/dec of sub-threshold slope boundary which is the theoretical limit at room temperature for thermionic injection based FETs. In the present invention, we exploit the presence of reservoirs for both types of carriers (holes and electrons) in an innovative way to realize single transistor DRAM operation.
In conventional metal-oxide-semiconductor (MOS) FET structure (p+/i/+p or n+/i+/n), the desired potential well is inherently available and it is exploited with floating body effect in Z-RAM [0]; however the majority carrier injection to the body requires techniques (i.e. hot carrier injection) which degrade reliability and the device's life time. In a TFET structure, the opposite doping of drain and source regions solves this problem and the challenge of carrier injection becomes one of creation and preservation of a potential well to store the charges.
Tunnelling FETs which are essentially p/i/n structures can be exploited both as P type (PTFET) or N type (NTFET) devices. In both modes of operation (P or N) p-n is reverse-biased. Conventionally, in N type operation, p+ region is used as source, n+ region is used as drain and the gate terminal is biased positively with respect to source allowing electrons to tunnel from source to channel. Similarly for P type operation, p+ region is used as drain, n+ region is used as source and the gate terminal is biased negatively with respect to source allowing holes to tunnel from drain to channel region.